The sulphur amino acid, cysteine, plays an essential role in maintaining cellular redox potential and is a key constituent of the antioxidant, glutathione. Cysteine is highly reactive and readily oxidises to the disulfide form, cystine, producing oxygen radicals as a by-product. In this book, the authors present current research in the study of the biosynthesis, chemical structure and toxicity of cysteine. Topics include cysteine biosynthesis in plants; the structural and functional role of cysteine residues; new cysteine synthesis pathways and their applications; implications for cysteine neurotoxicity; and the protecting groups used to cage cysteine and their applications in biochemical experiments are reviewed.